Ferro paper (Blueprint Paper roll)

Blue print paper is a type of paper that is used in the process of creating architectural or engineering drawings through a technique called "blueprinting". The process involves creating a negative image of the original drawing on a photosensitive paper coated with a solution of potassium ferricyanide and ferric ammonium citrate. When exposed to light, the solution undergoes a chemical reaction that creates a blue image on the paper. The unexposed areas remain white.